Fluorescent organic compounds are important chemicals used as a luminescent material of organic light-emitting diodes and a fluorescent dye for biological fluorescence imaging. Therefore, numerous fluorescent organic compounds have been reported in the field of both basic and applied researches. In particular, dyes (for example, acrylodan) whose fluorescent color dramatically changes depending on the surrounding environment are practically used as fluorescent probes for site-specific visualization. It is often reported that the molecular design of combining a highly electron-donating (donor type) π-conjugated unit with a highly electron-accepting (acceptor type) π-conjugated unit is effective in obtaining such fluorescent characteristics. The luminescent color, luminescence efficiency of a fluorescent dye, and the degree of solvent effect highly depend on the choice of π-conjugated units as the donor and the acceptor. Recently, phosphole compounds have been attracting attention as a new type of electron-accepting π-conjugated unit. For example, NPL 1 reports synthesis examples of 2-alkenyl-benzo[b]phosphole oxide and 2-alkynyl-benzo[b]phosphole oxide and optical data thereof. Also, NPL 2 reports synthesis examples of 2-aryl-benzo[b]phosphole oxide and the electrochemical properties thereof. The following are examples of the compounds described in NPLs 1 and 2:
